The first cellular mobile radio systems in public use were generally analog systems for the transmission of speech or other analog information. The systems included a plurality of radio channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had comparably large coverage cells.
More recently, digital cellular mobile radio systems for public use have been designed. Digital cellular mobile radio systems have digital channels for transmitting digital or digitized analog information between base and mobile stations by transmitting digitally modulated radio signals. Digital cellular mobile radio systems offer substantial advantages over analog cellular mobile radio systems.
One digital mobile radio system intended to be used commonly in many European countries is the GSM system. In European countries already having an analog cellular mobile system, the new digital GSM system will be introduced independent of any existing analog system. Thus, the GSM system base and mobile stations are not designed to be compatible with existing analog systems.
Rather than introduce an independent digital cellular mobile radio system, like the GSM system, into an area with an existing analog cellular system, the present invention seeks to introduce a digital cellular mobile radio system designed for cooperation with an existing analog cellular mobile radio system. To obtain digital channels within the frequency band allotted to cellular mobile radio systems, a number of radio channels allotted to the existing analog mobile radio systems could be allocated for use in the digital cellular mobile radio system. One design of the digital mobile radio system allows three or possibly six digital channels to occupy the same frequency band of one previous analog radio channel by using time division multiplexing. Replacing some analog channels with digital channels in time division multiplex increases the total number of radio channels.
The digital cellular system could be introduced by gradually increasing the number of digital channels while decreasing the number of analog channels. Analog mobile stations already in use will continue to use the remaining analog channels. At the same time, digital mobile stations will use the new digital channels. Dual-mode mobile stations will be able to use both the remaining analog channels and the new digital channels.
Both analog and digital cellular systems must be able to generate DTMF (Dual Tone Multiple Frequency) tones. In the analog systems, the DTMF generator is part of the mobile station. The DTMF tones are generated in the mobile station and transferred as audio information over the radio channel through the cellular system to the other party of the call connection. That other party usually is a subscriber from the public switched telephone network (PSTN). DTMF tones may also be generated by the PSTN subscriber and detected in the mobile station.
The GSM digital system requires separate hardware components to generate DTMF tones. The present invention seeks to avoid adding hardware for DTMF tone generation by utilizing the existing the speech coder/decoder hardware to accomplish the generation and decoding of DTMF tones. As a result of coding DTMF tones into digital format, digital data messages rather than audio tones must be transferred to and from the mobile station. Consequently, the generation and detection of the DTMF tones are performed at the base station instead of the mobile station.
In cellular systems in the United States, the Electronic Industries Association specification standards EIT/TIA-IS-54 require DTMF tones to be transmittable in two modes: burst DTMF and continuous DTMF. In the burst DTMF mode, when a mobile station user dials, e.g., by depressing a push-button key, the dialed digit/symbol, e.g., corresponding to the depressed key, is stored in the mobile station. After dialing, so that a sequence of digits/symbols have been entered and stored at the mobile station, the mobile user may initiate the transmission of the dialed sequence of digits/symbols, e.g., by depressing a send key. The mobile station transmits a message including the sequence of digits/symbols. In response to receiving this message, the land-based system generates a DTMF pulse sequence including a pulse for each dialed digit/symbol separated from adjacent pulses by pulse pauses. The DTMF pulses have a uniform width, e.g., 95 msec, as have the pulse pauses, e.g., 60 msec.
In the continuous DTMF mode, a data message is initiated at the beginning and at the end of each dialing of a digit/symbol, e.g., at the depression and at the release of a key. In response to receiving such a data message, the land-based system starts and stops the generation of DTMF tones, respectively. In the continuous mode, a DTMF signal is not generated automatically for a fixed time, but continues for the time of actuation/depression of a key. Thus, in the continuous mode, the duration of the DTMF transmission is variable.
A problem arises in handoff situations where a handoff is to be accomplished during a DTMF transmission. In other words, a handoff order or request occurs immediately after an entry of a number/symbol sequence on a mobile radio or telephone handset but before the transmission of the DTMF tones corresponding to the number/symbol sequence is terminated. Specifically, the problem arises in switching the transmission links in the land-based system between the two base stations involved in a handoff, e.g., the serving base station and the target base station and the mobile service switching center(s) to which the base stations are connected at the same time DTMF tones are being transmitted from the responsible base station towards the other party. Although more than one mobile switching center may be involved in handoff situations, only one mobile switching center is illustrated for purposes of simplifying discussion.
A simple example illustrates the problem of coincidental occurrence of handoff and DTMF transmissions. When a DTMF message is sent from a mobile station to a base station, a maximum of 63 binary digits can be accommodated in a single message. Once the 63 DTMF digits are received in the associated base station and translated into the appropriate DTMF format, the translated DTMF tones are transmitted to the other party. This transmission may take up to about 10 seconds based on the fact that each digit/symbol and corresponding pause takes 95+60=155 msec per digit. If these are 63 digits in a message, the total time (63.times.155 msec) is approximately 10 seconds. If a handoff occurs during these 10 seconds, the DTMF transmission is easily disrupted. A similar situation arises in the continuous DTMF mode or when DTMF tones are sent from the PSTN via the mobile switching center MSC to a base station BS.
Prior U.S. Pat. No. 4,654,867 to Labedz et al discloses a cellular telephone system that halts data transmission on a first radio channel prior to a handoff. When handoff is complete, the data transmission resumes on a second radio channel. However, the Labedz system is not suited to handle the specific situation addressed in the present invention. The problem that is not addressed in Labedz and that is resolved by the present invention is the situation where the base station is transmitting a series of DTMF tones to the other party and a handoff is ordered during this transmission. Since the DTMF transmission from the base station BS to the other party may require up to 10 seconds, the interruption of that DTMF signal transmission for a handoff may introduce errors in that DTMF transmission.
The present invention resolves these problems by minimizing these handoff-related errors in the DTMF transmission.